The present invention relates to a technology of predicting a remaining capacity of a battery.
To roughly classify a conventional system for predicting a remaining capacity of a battery, two types of systems have hitherto been employed.
One type of predicting system is a current integration system that integrates charge/discharge currents and predicts a current capacity remaining in the battery, and this type of system is used in most cases generally when requiring accuracy.
In the current integration system, to begin with, at a point of time when detecting a full-charge state, a predetermined full-charge capacity is set in a remaining capacity. At first, the full-charge capacity is substituted with a related capacity.
When starting the discharge, the discharge currents are integrated, then a remaining capacity is obtained by subtracting the integrated current from the full-charge capacity, and a percentage is acquired at a ratio of the full-charge capacity to the remaining capacity.
For example, in the case of the battery of which the rated capacity is on the order of 2000 mAh, the remaining capacity is set to 2000 mAh when detecting the full-charge state. Then, in the case of conducting the discharge for 10 min when the current is on the order of 1.8 A, the remaining capacity is obtained such as 2000 mAh-1800 mA×10/60 (h)=1700 mAh, and the percentage of the remaining capacity is given by 1700/2000×100=85%.
Another system is an open-circuit voltage system that predicts the remaining capacity from the open-circuit voltage of the battery.
In the open-circuit voltage system, a table is generated by previously obtaining a relation between the open-circuit voltage and the remaining capacity of the battery, and the remaining capacity corresponding to the open-circuit voltage of the battery is obtained by referring to the table. Herein, the open-circuit voltage may be directly measured if the battery is in a state where neither the charge nor the discharge is conducted, however, during the charge/discharge process, the charge/discharge currents are measured, and the open-circuit voltage is obtained from information on internal impedance.
For instance, the open-circuit voltage (OCV) can be inferred by the following formula, where V is a voltage of the battery, I is a charge/discharge current, and Ri is internal impedance of the battery.OCV=V+Ri×I 
Moreover, for example, technologies disclosed in the following Patent documents 1-4 are given as the prior arts related to the invention of the present application.
[Patent Document 1]
    Japanese Patent Application Laid-Open Publication No. 11-223665[Patent Document 2]    Japanese Patent Application Laid-Open Publication No. 2005-331482[Patent Document 3]    Japanese Patent Application Laid-Open Publication No. 6-242193[Patent Document 4]    Japanese Patent Application Laid-Open Publication No. 8-179018